Biogeochemistry. The nitrogen fix.
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Atmosphere↗
Biomedical subjects
J L Siefert
Publications and source records attributed to J L Siefert.
A coupled ecosystem-climate model for predicting the methane concentration in the Archean atmosphere.
A simple coupled ecosystem-climate model is described that can predict levels of atmospheric CH4, CO2, and H2 during the Late Archean, given observed constraints on Earth's surface temperature. We find that methanogenic bacteria should have converted most of the available atmospheric H2 into CH4, and that CH4 may have been equal in importance to CO2 as a greenhouse gas. Photolysis of this CH4 may have produced a hydrocarbon smog layer that would have shielded the surface from solar UV radiation. Methanotrophic bacteria would have consumed some of the atmospheric CH4, but they would have been incapable of reducing CH4 to modern levels. The rise of O2 around 2.3 Ga would have drastically reduced the atmospheric CH4 concentration and may thereby have triggered the Huronian glaciation.
Aerobiosis↗
Phylogeny of marine Bacillus isolates from the Gulf of Mexico.
The phylogeny of 11 pigmented, aerobic, spore-forming isolates from marine sources was studied. Forty-two biochemical characteristics were examined, and a 16S rDNA sequence was obtained for each isolate. In a phylogenetic tree based on 16S sequencing, four isolates (NRRL B-14850, NRRL B-14904, NRRL B-14907, and NRRL B-14908) clustered with B. subtilis and related organisms; NRRL B-14907 was closely related to B. amyloliquefaciens. NRRL B-14907 and NRRL B-14908 were phenotypically similar to B. amyloliquefaciens and B. pumilus, respectively. Three strains (NRRL B-14906, NRRL B-14910, and NRRL B-14911) clustered in a clade that included B. firmus, B. lentus, and B. megaterium. NRRL B-14910 was closely related phenotypically and phylogenetically to B. megaterium. NRRL B-14905 clustered with the mesophilic round spore-producing species, B. fusiformis and B. sphaericus; the isolate was more closely related to B. fusiformis. NRRL B-14905 displayed characteristics typical of the B. sphaericus-like organisms. NRRL B-14909 and NRRL B-14912 clustered with the Paenibacillus species and displayed characteristics typical of the genus. Only NRRL B-14851, an unusually thin rod that forms very small spores, may represent a new Bacillus species.
Bacillus↗
Co-conservation of rRNA tetraloop sequences and helix length suggests involvement of the tetraloops in higher-order interactions.
Terminal loops containing four nucleotides (tetraloops) are common in structural RNAs, and they frequently conform to one of three sequence motifs, GNRA, UNCG, or CUUG. Here we compare available sequences and secondary structures for rRNAs from bacteria, and we show that helices capped by phylogenetically conserved GNRA loops display a strong tendency to be of conserved length. The simplest interpretation of this correlation is that the conserved GNRA loops are involved in higher-order interactions, intramolecular or intermolecular, resulting in a selective pressure for maintaining the lengths of these helices. A small number of conserved UNCG loops were also found to be associated with conserved length helices, consistent with the possibility that this type of tetraloop also takes part in higher-order interactions.
Base Sequence↗
Conserved gene clusters in bacterial genomes provide further support for the primacy of RNA.
Five complete bacterial genome sequences have been released to the scientific community. These include four (eu)Bacteria, Haemophilus influenzae, Mycoplasma genitalium, M. pneumoniae, and Synechocystis PCC 6803, as well as one Archaeon, Methanococcus jannaschii. Features of organization shared by these genomes are likely to have arisen very early in the history of the bacteria and thus can be expected to provide further insight into the nature of early ancestors. Results of a genome comparison of these five organisms confirm earlier observations that gene order is remarkably unpreserved. There are, nevertheless, at least 16 clusters of two or more genes whose order remains the same among the four (eu)Bacteria and these are presumed to reflect conserved elements of coordinated gene expression that require gene proximity. Eight of these gene orders are essentially conserved in the Archaea as well. Many of these clusters are known to be regulated by RNA-level mechanisms in Escherichia coli, which supports the earlier suggestion that this type of regulation of gene expression may have arisen very early. We conclude that although the last common ancestor may have had a DNA genome, it likely was preceded by progenotes with an RNA genome.
Archaea↗
Phylogenetic position of the genus Hydrogenobacter.
The genus Hydrogenobacter consists of extremely thermophilic, obligately chemolithotrophic organisms that exhibit anaerobic anabolism but aerobic catabolism. Preliminary studies of the phylogenetic position of these organisms based on limited 16S ribosomal DNA sequence data suggested that they belong to one of the earliest branching orders of the Bacteria. In this study, the complete 16S ribosomal DNA sequences of two type strains, Hydrogenobacter thermophilus TK-6 and Calderobacterium hydrogenophilum Z-829, and another isolate, Hydrogenobacter sp. strain T3, were determined, and the phylogenetic positions of these organisms were examined. Our results revealed that the two type strains are members of a single genus, the genus Hydrogenobacter. Our results also verified the previous conclusion that the Aquifex-Hydrogenobacter complex belongs to a very early branching order, the "Aquificales." Within this order, the relationships among the various organisms are such that only a single family, the "Aquificaceae," can be recognized at this time. Given the early branching point of the "Aquificales," the characteristics of these organisms support the view that the last common ancestor of existing life was thermophilic and suggest that this ancestor may have fixed carbon chemoautotrophically.
Bacteria↗